It sounds very strange... I've tested also server errors and everything was fine... I have no other idea but to step int the code, both extension and Ext, to see what's happening.

You know what was wrong?

I was using an upload script hosted on a different virtual host to the one I was running the upload panel from. As soon as I moved the script to the same virtual host I started to receive valid values in the upload callback. Although quite why that should have made any difference I am not entirely sure.